The role of the thyroid hormones thyroxine and triiodothyronine in the regulation of protein metabolism in vertebrate liver will be studied in relation to late fetal development and birth and to metabolic adaptation in the adult. The principal experimental approach involves determination of rates of processes involved in the pathways of protein synthesis and secretion by rapid kinetic measurements in vivo. Polypeptide chain assembly time will be determined as a function of gestational age in liver of fetal rat in utero and in early neonatal life. Correlative studies of enzymes of the protein synthetic pathway from liver, particularly elongation factor I, will be made in these animals and in adult rats of differing thyroid status: euthyroid, hyperthyroid and surgically thyroidectomized. Further purification and characterization of phenylalanyl-tRNA synthetases from rat and toadfish liver will be made. The capacity for variation of body temperature in toadfish will be utilized in the study of reaction mechanisms and control points in protein metabolism in vivo. Kinetics and temperature dependency will be studied for the processes of amino acid uptake by liver, formation of aminoacyl-tRNA, polypeptide chain assembly and secretion of plasma proteins by liver.